This application based on application Nos. 257406/1998 and 52332/1999 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a pigment used in ink and the like and a process for producing the same, as well as water base ink using such a pigment and a process for producing the same. In particular, the present invention is characterized in that it enables easy improvement of dispersion stability of the pigment in an aqueous medium.
2. Description of the Prior Art
As ink for ink recording apparatus such as an inkjet printer, water base ink, in addition to oil base ink, has been conventionally used from the viewpoint of easiness of handling and safety.
The water base ink widely used in the art generally employs a water-soluble dye as a colorant which is dissolved in an aqueous medium.
However, the water base ink using a water-soluble dye has poor water resistance. Accordingly, there have been problems that, when supplied to a recording medium such as a plain paper sheet for image production, the ink produces blurring called bleeding or feathering and that the ink is caused to run by water contacting therewith.
On this account, the recent studies focus on using a pigment such as carbon black as a colorant for ink, and using water base ink in which the pigment such as carbon black are dispersed in an aqueous medium.
However, the pigment such as carbon black generally presents poor dispersion stability in an aqueous medium. Accordingly, when the water base ink is used in an ink recording apparatus such as an inkjet printer, there are some problems. For example, the pigment such as carbon black in the water base ink gradually forms agglomeration, which clogs a nozzle of the ink recording apparatus, resulting in reduced ink ejection stability. This leads to unstable image production or variations in the image density. As a result, stable production of favorable images is not ensured.
Therefore, in order to improve dispersion stability of the pigment such as carbon black in water base ink, adding a dispersing agent to the water base ink has been conventionally attempted. Examples of such a dispersing agent include a polyvalent alcohol alkyl ether derivative such as triethylene glycol monobutyl ether or a polyvalent alcohol ester derivative such as glyceryl monoacetate, and so on.
However, when any one of the above-mentioned dispersing agents is added to the water base ink in excess, the ink becomes liable to clog a nozzle of the ink recording apparatus by this dispersing agent, resulting in reduced ink ejection stability. Further, the excessive addition of the dispersing agent to the ink also leads to a reduced pigment concentration in the ink. This results in lowered image densities.
In recent years, a method has been proposed aiming at an improvement of dispersion stability of carbon black in an aqueous medium by oxidizing the carbon black with a hypohalogenous acid salt, as disclosed in Japanese Patent Laid-Open No. 319444/1996. Also, a method has been proposed aiming at an improvement of dispersion stability of carbon black in an aqueous medium by chemically binding hydrophilic groups such as --C.sub.6 H.sub.5 COOH, --C.sub.6 H.sub.5 SO.sub.3 H on a surface of the carbon black by a diazonium salt, as disclosed in U.S. Pat. No. 5,554,739.
However, in the method disclosed in Japanese Patent Laid-Open No. 319444/1996, the treatment therein takes a very long time because, in order to oxidize the carbon black with the hypohalogenous acid salt as mentioned above, the aqueous medium, the carbon black and the hypohalogenous acid are mixed and heated to effect oxidation of the carbon black. Similarly, in the method disclosed in U.S. Pat. No. 5,554,739, the reaction steps in chemically binding the above-mentioned hydrophilic groups on the surface of the carbon black are complicated and take a long time. Thus, in both cases, the problem remains that treatments of the carbon black take long time and it is impossible to sufficiently improve the dispersion stability of the carbon black in the aqueous medium within a short time.